1. Field of the Invention
The present invention relates generally to the field of thermal imaging systems. More specifically, the present invention discloses a thermal imaging system for viewing within the cylinder of an internal combustion engine.
2. Statement of the Problem
In designing and testing internal combustion engines, it is advantageous to know the temperature profile of the various surfaces within the cylinder, including the cylinder head and valves, while the engine is in operation. For example, thermal images from within the engine cylinder can be used to detect localized hot spots and to study combustion chamber deposits.
Ideally, a thermal imaging system should be suitable for use with a wide variety of actual production engines, and should not be limited to a specially constructed engine used for testing. In addition, the system should not require extensive modifications to the engine to accommodate the imaging system. Extensive modification of the engine increases the risk that the image data generated by the system does not accurately reflect conditions within an actual production engine.
It is also important to be able to quickly and easily remove those portions of the imaging system that are exposed within the engine cylinder for the purpose of cleaning. Films and combustion deposits tend to accumulate rapidly on the exposed optical elements of the system. In particular, it should not be necessary to disassemble the entire engine to clean or replace the optical train.
A number of optical sensors for internal combustion engines have been invented in the past, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Linder et al. 4,377,086 Mar. 22, 1983 Muller 4,393,687 Jul. 19, 1983 Linder et al. 4,422,323 Dec. 27, 1983 Franke et al. 4,425,788 Jan. 17, 1984 Hattori et al. 4,444,043 Apr. 24, 1984 Boning et al. 4,446,723 May 8, 1984 Dils 5,052,214 Oct. 1, 1991 Remboski et al. 5,067,463 Nov. 26, 1991 Dils 5,099,681 Mar. 31, 1992 Remboski et al. 5,113,828 May 19, 1992 Kubota et al. 5,195,359 Mar. 23, 1993 Kubota et al. Japan 3-48742 July 18, 1989 ______________________________________
Zhao et al., "The Cylinder Head Temperature Measurement by Thermal Imaging Technique," SAE Technical Paper Series 912404 (October 1991)
The patents of Linder et al. show two optical sensors for combustion chambers. Both apparently use a photodetector (e.g., a photo diode) to monitor the overall luminosity of the combustion process. The patents of Linder et al. are representative of a large body of prior art in the field of "knock detectors" that are concerned with monitoring overall luminosity during the combustion cycle, rather than providing a thermal image of the interior of the combustion chamber. The Linder '323 patent shows an optical window that can be removed for cleaning.
Muller et al. describe an optical sensor incorporated in a spark plug housing for use in a knock sensor. The optical pickup is a quartz glass rod that detects the oscillations of the fuel-air mixture as variations in brightness. The end of the glass rod is located centrally between the spark gaps. No image is provided.
Franke et al. disclose a combustion monitoring system for a multi-cylinder engine to detect knocking. Each cylinder is equipped with fiber optics to direct light from within the cylinder to a photo diode to measure overall luminosity. Here again, no image is provided.
Hattori et al. disclose another example of the knock detector for internal combustion engines. An illumination detector 10 is threaded through the cylinder head. The illumination detector is made of quartz glass or sapphire (column 3, line 16 and column 4, lines 61-65).
Boning et al. disclose a further optical sensor incorporated in a spark plug housing for use in a knock sensor. Here again, the optical pickup is a quartz glass rod. The end of the rod facing the combustion chamber is coated with a layer of graphite 13.
The Dils patents discuss a knock detector using an optical fiber and a black body emitter to sense variations in heat flow within the cylinder.
The two patents of Remboski et al. disclose an internal combustion engine having a luminosity detector. The system controls operation of the engine based in part on the luminosity signal. The optical probe 19 is made of synthetic sapphire (column 4, lines 43-46 of the '828 patent).
Kubota et al. disclose another example of an optical system for detecting knocking in an internal combustion engine. The system uses a sapphire rod 1 (or 22) with a black body 2 (or 23) covering the end of the rod within the combustion chamber to measure heat flux.
The SAE paper by Zhao et al. discloses a thermal imaging system for measuring the temperature profile of the cylinder head of an internal combustion engine. A schematic drawing of this system is provided in FIG. 3 of the SAE paper. The system requires a specially modified engine cylinder and piston having a window in the side wall of the cylinder, a window in the side wall of the piston, a 45.degree. mirror within the piston, and a silicon window that replaces the head of the piston. The cylinder head is painted black to provide a high and uniform emissivity across its surface. In addition, a number of thermocouples are mounted onto the cylinder head, as shown in FIG. 4 of the SAE paper, to provide reference temperature readings. It should also be noted that the camera is only able to view the mirror and cylinder head during those portions of the combustion cycle when the windows in the cylinder wall and piston wall are in vertical alignment.
3. Solution to the Problem
None of the prior art references uncovered in the search show a system for viewing thermal images from within the combustion chamber (as opposed to measuring overall luminosity) using a removable optical probe that is inserted through the wall of the cylinder or as an integral part of a spark plug. In addition, none of the prior art references disclose: (1) use of polycrystalline spinel in an optical probe; (2) use of a black body/thermocouple within the field of view of the optical probe for temperature calibration; or (3) use of a side-viewing optical probe.